Sapphire substrate and semiconductor light emitting device

ABSTRACT

A sapphire substrate having a principal surface for growing a nitride semiconductor to form a nitride semiconductor light emitting device comprises a plurality of projections on the principal surface. Each of the projections has a bottom that has a substantially polygonal shape. Each side of the bottom of the projections has a depression in its center. Vertexes of the bottoms of the respective projections extend in a direction that is within a range of ±10 degrees of a direction that is rotated clockwise by 30 degrees from a crystal axis “a” of the sapphire substrate.

TECHNICAL FIELD

The present invention relates to a sapphire substrate for a nitridesemiconductor light emitting device, and a semiconductor light emittingdevice.

BACKGROUND ART

For example, a light emitting diode (LED) made of a nitridesemiconductor is usually constituted by sequentially laminating ann-type semiconductor layer, an active layer and a p-type semiconductorlayer on a sapphire substrate. In this light emitting diode, emittedlight is extracted from the side opposite the sapphire substrate or theside of the sapphire substrate, while light emitted from the activelayer is also radiated in a direction opposite the light emission side.Therefore, it becomes necessary to improve external quantum efficiencyby enabling light radiated in a direction opposite the light emissionside to extract effectively from the light emission side.

For example, Patent Document 1 discloses that external quantumefficiency is improved by arranging a plurality of projections havingtruncated triangular pyramid-shaped on a sapphire substrate. PatentDocument 1 also describes that generation of voids and deterioration ofcrystallinity can be suppressed by enabling crystal growth on a surface,on which projections having a truncated triangular pyramid shaped areformed, through the projections.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-2008-177528

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, the present inventors have intensively studied and found that,in a light emitting diode composed of a nitride semiconductor grown on asurface including projections having a truncated triangular pyramidshape, it was not necessarily possible that light emitted in thedirection opposite the light emission side is efficiently extracted fromthe light emission side, and external quantum efficiency is sufficientlyimproved.

Thus, an object of the present invention is to provide a sapphiresubstrate for a nitride semiconductor light emitting device, that canconstitute a nitride semiconductor light emitting device havingexcellent light extraction efficiency.

Another object of the present invention is to provide a semiconductorlight emitting device having excellent light extraction efficiency.

Means for Solving the Problems

In order to achieve the above objects, the sapphire substrate accordingto the present invention is a sapphire substrate having a principalsurface for growing a nitride semiconductor to form a nitridesemiconductor light emitting device and comprising a plurality ofprojections on the principal surface, wherein an outer periphery of abottom surface of each of the projections has at least one depression.This depression does not mean a depression in a vertical direction to asapphire substrate, but means a depression in a horizontal direction.

The plurality of projections are arranged so that a straight line passesthrough the inside of at least any one of projections when the straightline is drawn at any position in any direction in a plane including thebottom surfaces of the plurality of projections.

The projections have a substantially polygonal cone shape orsubstantially polygonal truncated pyramidal shape whose bottom surfacehas a substantially polygonal shape, respectively, and each side of thebottom surface includes a depression in the center.

The semiconductor light emitting device according to the presentinvention is characterized in that a nitride semiconductor layer lightemitting device is formed by growing a nitride semiconductor on oneprincipal surface of the sapphire substrate according to the presentinvention.

Effects of the Invention

The sapphire substrate constituted as described above according to thepresent invention has a principal surface comprising a plurality ofprojections for growing a nitride semiconductor to form a nitridesemiconductor light emitting device and an outer periphery of a bottomsurface of each of the projections has at least one depression.Therefore, light emitted in parallel to the sapphire substrate on asurface thereof is irradiated on projections, and thus it is possible toprovide a sapphire substrate for a nitride semiconductor light emittingdevice, that can constitute a nitride semiconductor light emittingdevice having excellent light extraction efficiency.

In the sapphire substrate, the plurality of projections are arranged sothat a straight line passes through the inside of at least any one ofprojections when the straight line is drawn at any position in anydirection in a plane including a bottom surface of the plurality ofprojections. Therefore, when a nitride semiconductor light emittingdevice is constituted on one principal surface, light, that propagatesin parallel and proximal to one principal surface, is reflected by atleast one of projections even if light propagates in any direction.

Accordingly, the sapphire substrate according to the present inventioncan provide a sapphire substrate for a nitride semiconductor lightemitting device, that can constitute a nitride semiconductor lightemitting device having excellent light extraction efficiency.

In the sapphire substrate, each of the projections has a substantiallypolygonal cone shape or substantially polygonal truncated pyramidalshape whose bottom surface has a substantially polygonal shape, and eachside of the bottom surface includes a depression in the center.Therefore, it is possible to easily realize the arrangement ofprojections so that a straight line passes through the inside of atleast any one of projections when the straight line is drawn at anyposition in any direction in a plane including a bottom surface of theplurality of projections.

In the light emitting device according to the present invention, anitride semiconductor layer light emitting device is formed by growing anitride semiconductor on one principal surface of the sapphire substrateaccording to the present invention. Therefore, it is possible to providea semiconductor light emitting device having excellent light extractionefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a nitride semiconductor light emittingdevice according to the embodiment of the present invention.

FIG. 2 is a plan view showing the arrangement of projections in asapphire substrate according to Embodiment 1 of the present invention.

FIG. 3 is a plan view showing the arrangement of projections in asapphire substrate according to Embodiment 2 of the present invention.

FIG. 4 is a plan view showing the arrangement of projections in asapphire substrate according to Embodiment 3 of the present invention.

FIG. 5 is a plan view showing the arrangement of projections in asapphire substrate according to Embodiment 4 of the present invention.

FIG. 6 is a plan view showing the arrangement of projections in asapphire substrate according to Embodiment 5 of the present invention.

FIG. 7 is a plan view showing the arrangement of projections in asapphire substrate according to Embodiment 6 of the present invention.

FIG. 8 is a plan view showing an example of a mask used when projectionsaccording to Embodiment 7 of the present invention is formed on asapphire substrate.

FIG. 9 is a plan view showing the constitution and arrangement ofprojections in a sapphire substrate according to Embodiment 7 of thepresent invention.

FIG. 10 is a plan view showing the constitution and arrangement ofprojections in a sapphire substrate according to Variation 1 of thepresent invention.

FIG. 11 is a plan view showing the constitution and arrangement ofprojections in a sapphire substrate according to Variation 2 of thepresent invention.

FIG. 12 is a plan view showing the constitution and arrangement ofprojections in a sapphire substrate according to Embodiment 8 of thepresent invention.

FIG. 13 is a plan view of a mask used when projections according toEmbodiment 8 of the present invention is formed on a sapphire substrate.

FIG. 14 is a plan view showing the constitution and arrangement ofprojections in a sapphire substrate according to Embodiment 9 of thepresent invention.

FIG. 15A is a plan view showing an orientation flat (plane A) and acrystal axis (axis a) of a sapphire wafer, and FIG. 15B is a plan viewshowing a direction of projections.

FIG. 16A is a plan view showing the constitution and arrangement ofprojections in a sapphire substrate according to Embodiment 10 of thepresent invention.

FIG. 16B is a plan view enlargely showing the constitution ofprojections of Embodiment 10.

FIG. 17 is a plan view showing an example of a mask used whenprojections according to Embodiment 10 of the present invention isformed on a sapphire substrate.

FIG. 18A is a plan view showing an orientation flat (plane A) and acrystal axis (axis a) of a sapphire wafer, FIGS. 18B and 18C are planviews schematically showing a direction of projections and a growth rateof a nitride semiconductor, and FIG. 18D is a schematic view showingcrystal orientation dependence of sapphire of a growth rate of a nitridesemiconductor.

FIG. 19 is a plan view showing the constitution and arrangement ofprojections in a sapphire substrate according to Embodiment 11 of thepresent invention.

FIG. 20 is a plan view of a mask used when projections according toEmbodiment 11 of the present invention is formed on a sapphiresubstrate.

FIG. 21 is a plan view showing the constitution and arrangement ofprojections in a sapphire substrate according to Embodiment 12 of thepresent invention.

FIG. 22 is a plan view of a mask used when projections according toEmbodiment 12 of the present invention is formed on a sapphiresubstrate.

MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1, the nitride semiconductor light emitting deviceaccording to the embodiment of the present invention is provided with asemiconductor laminated structure 20 in which a base layer 21, a firstconductive layer (n-type layer) 22, an active layer (emissive layer) 23and a second conductive layer (p-type layer) 24 are sequentiallylaminated on a sapphire substrate 10, and a plurality of projections(dimples) 1, each having a pyramidal shape or a trapezoidal conicalshape, are provided on a surface of the substrate 10 on which the baselayer 21 is grown.

Herein, particularly in the present embodiment, a plurality ofprojections 1 are arranged so that light, that propagates in parallel tothe surface of the substrate located between adjacent projections 1 andproximal to the surface of the substrate, is reflected by at least oneof projections even if light propagates in any direction.

Whereby, the nitride semiconductor light emitting device of the presentembodiment enables light emitted in the emissive layer 23 to efficientlyreflect in the light emission direction without being emitted from theside by a plurality of projections 1, and thus light extractionefficiency can be increased.

Specific arrangement example of projections 1 in a sapphire substrateaccording to the embodiment of the present invention will be describedbelow.

In the present invention, the projection 1 may have a substantiallyn-sided pyramidal shape, or a substantially truncated n-sided pyramidalshape including a top surface 1 b that is substantially parallel to abottom surface and has a shape substantially similar to that of thebottom surface, and each bottom surface may have a substantially n-sidedpolygon shape including n sides, each having an outwardly bulgedcircular arc shape. In the following descriptions of Embodiments 1 to 6,description is made using an example of the projection 1 having atypical substantially triangular pyramidal shape or a substantiallytruncated triangular pyramidal shape.

In the drawings to be referred in Embodiments 1 to 6, a planar shape (ashape of a bottom surface) of the projection 1 is schematically shown byregular triangle. When a projection 1 having a substantially triangularpyramidal shape or a substantially truncated triangular pyramidal shapeis formed by etching a sapphire substrate, each side of a bottom surfacethereof usually becomes a side having an outwardly bulged circular arcshape and an inclined side of the projection 1 becomes an outwardlybulged surface.

Embodiment 1

In a plan view of FIG. 2, the arrangement of projections 1 in a sapphiresubstrate according to the embodiment 1 of the present invention isshown.

In the present Embodiment 1, a plurality of projections 1, each having asubstantially triangular pyramidal shape, are arranged so that gravityof a bottom surface agrees with each lattice point of a triangularlattice having a lattice shape of regular triangle. In Embodiment 1,projections 1 are arranged on the lattice point of a triangular latticeso that all projections 1 face the same direction. In the presentdescription, the direction of projections 1 refers to the direction inwhich a line, that bisect an angle of a vertex of a bottom surface,faces outside from the inside of projections 1, and two or moreprojections 1 facing the same direction refers that all directionscorresponding between projections 1 face the same direction. In otherwords, it refers that projections are arranged in a state whereprojections can be superposed on a bottom surface of other allprojections 1 by parallel translation without rotating the bottomsurface.

The plurality of projections 1 thus arranged of Embodiment 1 arearranged so as to form a plurality of rows, and are respectivelyarranged so that a bisector that bisects an angle of one of vertexes ofprojections 1 arranged in the same line is located on a straight line,and also the projection direction defined by the direction, in which thebisector faces outside from the inside of projections 1, becomes thesame, even in case of looking at the row in any direction.

In the present Embodiment 1, in addition to the above arrangement, thesize of projections 1 is set so that a straight line necessarily passesthrough the inside of any one of projections 1 when the straight line isdrawn at any position in any direction in the plane including a bottomsurface of projections 1.

Using a sapphire substrate including a plurality of projections 1 havingthe size set as described above and arranged as shown in FIG. 2, when asemiconductor laminated structure 20, in which a base layer 21, a firstconductive layer (n-type layer) 22, an active layer (emissive layer) 23and a second conductive layer (p-type layer) 24 are sequentially formedon one principal surface on which plurality of projections 1 are formed,is constituted and thus a nitride semiconductor light emitting device isconstituted, light emitted on the sapphire substrate side among lightemitted in the emissive layer 23 is reflected by at least one ofprojections 1, even if light L1, that propagates in parallel to thesurface of the substrate located between adjacent projections 1 andproximal to the surface of the substrate, propagates in any direction.

Accordingly, the nitride semiconductor light emitting device constitutedusing the sapphire substrate of the present Embodiment 1 enables lightto efficiently reflect in the light emission direction without beingemitted from the side by the presence of the above arranged plurality ofprojections 1, and thus light extraction efficiency can be increased.

In the sapphire substrate of the present Embodiment 1, projections 1 arearranged comparatively densely by the above arrangement and theproportion occupied by the plurality of projections 1 provided on oneprincipal surface based on the entire one principal surface of thesapphire substrate increases, and thus enabling the growth of a nitridesemiconductor with low dislocation, resulting in high luminousefficiency of the nitride semiconductor light emitting device.

Embodiment 2

In a plan view of FIG. 3, the arrangement of projections 1 in a sapphiresubstrate according to Embodiment 2 of the present invention is shown.

The arrangement of FIG. 3 is composed of a plurality of rows that arerespectively constituted by a plurality of projections 1. The pluralityof projections 1 arranged in the same row are respectively arranged sothat a bisector that bisects an angle of one of vertexes of theprojections is located on a straight line and also the projectiondirection defined by the direction, in which the bisector faces outsidefrom the inside of projections 1, becomes the same. The arrangementdirection of projections 1 of odd numbered rows is opposite to thearrangement direction of projections 1 of even numbered rows. That is,the direction of projections 1 in each column is opposite betweenadjacent columns.

The size of a bottom surface of the respective projections 1 is set sothat a straight line passes through the inside of any one of projections1 even if the straight line is drawn at any position in parallel to therow and the straight line is drawn at any position in parallel to thecolumn.

Using a sapphire substrate including a plurality of projections 1 havingthe size set as described above and arranged as shown in FIG. 3, when asemiconductor laminated structure 20, in which a base layer 21, a firstconductive layer (n-type layer) 22, an active layer (emissive layer) 23and a second conductive layer (p-type layer) 24 are sequentially formedon one principal surface on which plurality of projections 1 are formed,is constituted and thus a nitride semiconductor light emitting device isconstituted, light emitted on the sapphire substrate side among lightemitted in the emissive layer 23 is reflected by at least one ofprojections 1, even if light L1, that propagates in parallel to thesurface of the substrate located between adjacent projections 1 andproximal to the surface of the substrate, propagates in any direction.

Accordingly, the nitride semiconductor light emitting device constitutedusing the sapphire substrate of the present Embodiment 2 enables lightto efficiently reflect in the light emission direction without beingemitted from the side by the presence of the above arranged plurality ofprojections 1, and thus light extraction efficiency can be increased.

In the sapphire substrate of the present Embodiment 2, projections 1 arearranged comparatively densely by the above arrangement and the areaoccupied by the bottom surface of the plurality of projections 1provided on one principal surface based on the entire one principalsurface of the sapphire substrate increases, and thus enabling thegrowth of a nitride semiconductor with low dislocation, resulting inhigh luminous efficiency of the nitride semiconductor light emittingdevice.

Embodiment 3

In a plan view of FIG. 4, the arrangement of projections 1, each havinga substantially triangular pyramidal shape, in a sapphire substrateaccording to Embodiment 3 of the present invention is shown. In thepresent Embodiment 3, a group G1 composed of six projections 1 as aplurality of projections 1, arranged so that gravity of a bottom surfaceof each projection agrees with a vertex and a center of regularpentagon, is repeatedly arranged in accordance with a fixed rule.

In the present Embodiment 3, regular pentagon in which six projections 1are arranged, is arranged in a sense (the same sense) that enablessuperposition on the other regular pentagon by parallel translationwithout rotation.

Regular pentagon is arranged so that the center of each regular pentagonagrees with a vertex of a triangular lattice point, and also projections1 are separated and do not overlap between adjacent groups G1.

Furthermore, regarding the size of each projection 1, the size of thebottom surface of each projection 1 is set so that a straight linepasses through the inside of any one of projections 1 when the straightline is optionally drawn on one principal surface.

Using a sapphire substrate including a plurality of projections 1 havingthe size set as described above and arranged as shown in FIG. 4, when asemiconductor laminated structure 20, in which a base layer 21, a firstconductive layer (n-type layer) 22, an active layer (emissive layer) 23and a second conductive layer (p-type layer) 24 are sequentially formedon one principal surface on which plurality of projections 1 are formed,is constituted and thus a nitride semiconductor light emitting device isconstituted, light emitted on the sapphire substrate side among lightemitted in the emissive layer 23 is reflected by at least one ofprojections 1, even if light L1, that propagates in parallel to thesurface of the substrate located between adjacent projections 1 andproximal to the surface of the substrate, propagates in any direction.

Accordingly, the nitride semiconductor light emitting device constitutedusing the sapphire substrate of the present Embodiment 3 enables lightto efficiently reflect in the light emission direction without beingemitted from the side by the presence of the above arranged plurality ofprojections 1, and thus light extraction efficiency can be increased.

In the sapphire substrate of the present Embodiment 3, projections 1 arearranged comparatively densely by the above arrangement and the areaoccupied by the bottom surface of the plurality of projections 1provided on one principal surface based on the entire one principalsurface of the sapphire substrate increases, and thus enabling thegrowth of a nitride semiconductor with low dislocation, resulting inhigh luminous efficiency of the nitride semiconductor light emittingdevice.

Embodiment 4

In a plan view of FIG. 5, the arrangement of projections 1 in a sapphiresubstrate according to the embodiment 4 of the present invention isshown. The sapphire substrate of the present Embodiment 4 is the same asin Embodiment 3 in that it includes a plurality of groups G1 composed ofsix projections 1 arranged on the vertex and center of regular pentagon,but is different in the rule when the group G1 is repeatedly arranged.

That is, in the sapphire substrate of Embodiment 4, as shown in FIG. 5,a plurality of groups G1 arranged in the first row (uppermost row in thedrawing) are juxtaposed so that regular pentagon, that determines thearrangement of projections 1, becomes the same sense.

A plurality of groups G1 arranged in the subsequent second row arejuxtaposed so that regular pentagon, that determines the arrangement ofeach group G1, faces opposite to regular pentagon, that determines thearrangement of the group G1 arranged in the first row, relative to thegroup Cl arranged in the first row.

At this time, the arrangement position of the group G1 arranged in thefirst row and that of the group G1 arranged in the second row are set sothat the center of regular pentagon, that determines the arrangement ofthe group Cl arranged in the first row, and the center of regularpentagon, that determines the arrangement of the group G1 arranged inthe second row, are located at the vertex of triangular lattice.

Furthermore, as shown in FIG. 5, the arrangement in the third row andthat of the fourth row are set so that the third row and the fourth rowbecome linear symmetry. Hereinafter, the arrangement of projections 1 inthe sapphire substrate of Embodiment 4 is set by repeating thearrangement similar to that in the first row to fourth row.

Similar to Embodiment 3, regarding the size of each projection 1, thesize of the bottom surface of each projection 1 is set so that astraight line passes through the inside of a bottom surface of at leastone of projections 1 when the straight line is optionally drawn on oneprincipal surface.

The nitride semiconductor light emitting device thus constituted using asapphire substrate of the present Embodiment 4 including a plurality ofprojections 1 having the size set as described above and arranged asshown in FIG. 5 enables light to efficiently reflect in the lightemission direction without being emitted from the side by a plurality ofprojections 1, and thus light extraction efficiency can be increased.

In the sapphire substrate of the present Embodiment 4, projections 1 arearranged comparatively densely by the above arrangement and the areaoccupied by the bottom surface of the plurality of projections 1provided on one principal surface based on the entire one principalsurface of the sapphire substrate increases, and thus enabling thegrowth of a nitride semiconductor with low dislocation, resulting inhigh luminous efficiency of the nitride semiconductor light emittingdevice.

Embodiment 5

In a plan view of FIG. 6, the arrangement of projections 1 in a sapphiresubstrate according to Embodiment 5 of the present invention is shown.In the present Embodiment 5, a group G1 composed of six projections 1 asa plurality of projections 1 arranged so that gravity of a bottomsurface of each projection agrees with a vertex and a center of regularpentagon is repeatedly arranged in accordance with a fixed rule that isdifferent from in Embodiments 3 and 4.

In the present Embodiment 5, the group G1 arranged in the same column isarranged in the same direction without rotating, and the group G1 isarranged between adjacent columns in a reverse sense.

The group G1 arranged in even numbered columns is arranged slightlydownward as compared with the group G1 arranged in add numbered columns.As a result, the sense of the group G1 arranged in each row becomesreverse between adjacent rows, and a line connecting centers of regularpentagons, that determine the arrangement of the group G1 in each row,becomes a bent line (zigzag line).

Furthermore, regarding the size of each projection 1, the size of thebottom surface of each projection 1 is set so that a straight linepasses through the inside of any one of projections 1 when the straightline is optionally drawn on a plane including a bottom surface ofprojection 1.

Using a sapphire substrate including a plurality of projections 1 havingthe size set as described above and arranged as shown in FIG. 6, when asemiconductor laminated structure 20, in which a base layer 21, a firstconductive layer (n-type layer) 22, an active layer (emissive layer) 23and a second conductive layer (p-type layer) 24 are sequentially formedon one principal surface on which plurality of projections 1 are formed,is constituted and thus a nitride semiconductor light emitting device isconstituted, light emitted on the sapphire substrate side among lightemitted in the emissive layer 23 is reflected by at least one ofprojections 1, even if light L1, that propagates in parallel to thesurface of the substrate located between adjacent projections 1 andproximal to the surface of the substrate, propagates in any direction.

Accordingly, the nitride semiconductor light emitting device constitutedusing the sapphire substrate of the present Embodiment 5 enables lightto efficiently reflect in the light emission direction without beingemitted from the side by the presence of the plurality of projections 1,and thus light extraction efficiency can be increased.

In the sapphire substrate of the present Embodiment 5, the area occupiedby the bottom surface of the plurality of projections 1 provided on oneprincipal surface based on the entire one principal surface of thesapphire substrate increases, and thus enabling the growth of a nitridesemiconductor with low dislocation, resulting in high luminousefficiency of the nitride semiconductor light emitting device.

Embodiment 6

In a plan view of FIG. 7, the arrangement of projections 1 in a sapphiresubstrate according to Embodiment 6 of the present invention is shown.In the present Embodiment 6, a group G10, which is composed of eightprojections 1 as a plurality of projections 1 arranged respectively on avertex of regular octagon, and projections 2 composed of projectionslarger than the projections 1 arranged on the center of regular octagon,is repeatedly arranged in accordance with a fixed rule.

In the present Embodiment 6, the group G10 is repeatedly arranged in twodirections of a row and a column, that intersect with each other, andrespectively shares two projections 1 between adjacent two groups G10 inthe same row and between adjacent two groups G10 in the same column.

Furthermore, the size of each projection 1 and the size of eachprojection 2 are set so that a straight line passes through the insideof projections 1 or projections 2 when the straight line is optionallydrawn on one principal surface including a bottom surface of projections1.

Using a sapphire substrate including projections 1 and projections 2,each having the size set as described above, and also including aplurality of projections 1 arranged as shown in FIG. 7, when asemiconductor laminated structure 20, in which a base layer 21, a firstconductive layer (n-type layer) 22, an active layer (emissive layer) 23and a second conductive layer (p-type layer) 24 are sequentially formedon one principal surface on which plurality of projections 1 are formed,is constituted and thus a nitride semiconductor light emitting device isconstituted, light emitted on the sapphire substrate side among lightemitted in the emissive layer 23 is reflected by at least one ofprojections 1, even if light L1, that propagates in parallel to thesurface of the substrate located between adjacent projections 1 andproximal to the surface of the substrate, propagates in any direction.

Accordingly, the nitride semiconductor light emitting device constitutedusing the sapphire substrate of the present Embodiment 6 enables lightto efficiently reflect in the light emission direction without beingemitted from the side by the plurality of projections 1 and projections2, and thus light extraction efficiency can be increased.

In the sapphire substrate of the present Embodiment 6, the area occupiedby the bottom surface of the plurality of projections 1 provided on oneprincipal surface based on the entire one principal surface of thesapphire substrate increases, and thus enabling the growth of a nitridesemiconductor with low dislocation, resulting in high luminousefficiency of the nitride semiconductor light emitting device.

Embodiment 7

In a plan view of FIG. 9, the shape and the arrangement of projections 1in a sapphire substrate according to Embodiment 7 of the presentinvention are shown.

As shown in FIG. 9, a bottom surface and a top surface of projections ofEmbodiment 7 have a truncated triangular pyramidal shape modified asdescribed hereinafter. Therefore, it is possible to easily realize thearrangement of a plurality of projections 1, that enables light to beeffectively reflected by projections 1, even if light L1, thatpropagates in parallel and proximal to a surface of the substrate,propagates in any direction, and thus light extraction efficiency can beimproved.

<Shape of Projections 1>

The bottom surface of projections of Embodiment 7 has a substantiallytriangular shape in which each of sides 11, 12, 13 include a depressionin the center.

Specifically, the side 11 is respectively composed of outwardly flangedtwo curved lines 11 a, 11 b, and a depression is formed at theconnection portion thereof. Similarly, the side 12 is respectivelycomposed of outwardly flanged two curved lines 12 a, 12 b, and adepression is formed at the connection portion thereof. The side 13 isrespectively composed of outwardly flanged two curved lines 13 a, 13 b,and a depression is formed at the connection portion thereof.

The top surface of projections 1 of Embodiment 7 also has asubstantially triangular shape, that is almost similar to that of thebottom surface including a depression in the center of each side.

In projections 1 of Embodiment 7, on the inclined side between thebottom surface and the top surface, a ridge connecting a vertex of thetop surface to a vertex of the bottom surface, and a valley connectingthe deepest portion of a depression of the bottom surface to the deepestportion of a depression of the top surface are formed.

In Embodiment 7, projections 1 constituted as described above arearranged so that one of vertexes of a bottom surface of one projection 1is located (enter into) in a proximal region defined by connectingpoints of two vertexes of a bottom surface of the other projection 1 andthe deepest point of a depression existing therebetween, betweenadjacent projections.

Whereby, a straight line can necessarily pass through the inside of anyone of projections 1 when the straight line is drawn at any position inany direction in a plane including a bottom surface of projections 1.

In the present Embodiment 7, it is more preferred to arrange so that thedeepest portion of a depression of the other projection 1 is located onan extended line of a line that bisect an angle of a vertex of oneprojection 1 located in the proximal region.

It is also preferred that each of projections 1 has a shape with linearsymmetry to a straight line that connects a depression to a vertex tothe side including the depression, and thus the above arrangement can beeasily realized.

When a nitride semiconductor light emitting device is constituted usinga sapphire substrate of Embodiment 7 in which the shape and thearrangement of projections 1 are set as described above, it is possibleto reflect by projections 1 even if light L1, that propagates inparallel and proximal to a surface of the substrate, propagates in anydirection.

Accordingly, the nitride semiconductor light emitting device constitutedusing a sapphire substrate of the present Embodiment 7 enables light toefficiently reflect in the light emission direction without beingemitted from the side by a plurality of projections 1, and thus lightextraction efficiency can be increased.

The area occupied by the bottom surface of the plurality of projections1 provided on one principal surface based on the entire one principalsurface in the sapphire substrate of the present Embodiment 7 increases,and thus enabling the growth of a nitride semiconductor with lowdislocation, resulting in high luminous efficiency of the nitridesemiconductor light emitting device.

Furthermore, in the sapphire substrate of the present Embodiment 7, forexample, the height of the projection 1 can be decreased as comparedwith the projection having a triangular pyramidal shape. Therefore, itbecomes possible to supply a raw gas, almost uniformly, to a growthsurface (surface of a sapphire substrate located between projections 1)of a nitride semiconductor layer when a base layer 21 is grown on oneprincipal surface on which a plurality of projections 1 are formed.

Whereby, it becomes possible to perform growth in a lateral direction,that uniformly covers projections 1 from the growth surface of a nitridesemiconductor layer, and to decrease the height of projections 1, andthus a flat surface is obtained by a comparatively thin base layer 21.

Projections 1 having a petal shape in the present Embodiment 7 can beformed by setting a crystal form of a substrate and orientation of asurface of a substrate on which projections 1 are formed, mask shape andsize, and etching conditions according to the objective shape.

In FIG. 8, an example of a mask M1 in case of forming projections 1shown in FIG. 9 on a C-plane of a sapphire substrate is shown. The maskM1 of this example is composed of three legs (tripod shape), each havingthe same length and the same width, extending in three directions thatchange every 120 degrees from the center.

Using such a mask M1 having a tripod shape, when the C-plane surface ofthe sapphire substrate is wet-etched with sulfuric acid or phosphoricacid, a top surface 1 b of projections 1 is formed, under the mask M1,into the shape in which the shape of the mask M1 right is modified, andan inclined side 1 a with an end of the top surface of projections 1 asa top end is formed.

That is, the shape of the top surface of projections 1 is influenced bydirection dependence of an etching rate due to a crystal form with theprogress of etching, and thus it becomes the shape in which the tripodshape of the mask M1 is modified depending on etching characteristics.Whereby, the shape becomes a substantially triangular shape thatincludes a depression in the center of each side and also includes asharpened vertex.

Similarly, the bottom surface of projections 1 also becomes asubstantially triangular shape, that is almost similar to the shape of atop surface including a depression in the center of each side and alsoincluding a sharpened vertex, in which the tripod shape of the mask M1is modified depending on etching characteristics.

Furthermore, due to direction dependence of the etching rate due to acrystal form, a ridge 1 r connecting a vertex of the top surface to avertex of the bottom surface, and a valley 12 connecting the deepestportion of a depression of the bottom surface to the deepest portion ofa depression of the top surface are formed on the inclined side betweenthe bottom surface and the top surface.

A ridge 1 r protruding outwardly from a leg tip is formed at the tipportion (leg tip) of the leg of the mask M1, and inclined surfaces, eachhaving a different inclination direction, are respectively formed onboth sides of the ridge 1 r. Accordingly, a plurality of masks M1 areformed so that a leg tip of one leg of the other mask M1 is proximal toor agrees with a straight line that connects angles of two leg tips ofone mask M1, between adjacent masks M1, whereby, a plurality ofprojections 1 are arranged so that one of vertexes of a bottom surfaceof one projection 1 is located (enter into) in a proximal region definedby connecting points of two vertexes of a bottom surface of the otherprojection 1 and the deepest point of a depression existingtherebetween, between adjacent projections.

The sense of projections 1 can be set to the desired sense by changingan off angle from a C-plane of a surface of a sapphire substrate onwhich projections 1 are formed.

While description was made by way of an example of the same arrangementof projections as in Embodiment 1 in the above Embodiment 7, thearrangement described in Embodiments 2 to 6 can also be applied.

Variation

While projections, each having a substantially polygonal pyramidal shape(particularly, substantially triangular pyramidal shape) whose bottomsurface has a substantially polygonal shape (particularly, substantiallytriangular shape), were described in the above Embodiments 1 to 7, thepresent invention is not limited thereto. The projections may be eitherprojections 31 whose bottom surface shape has a dogleg shape bent in thecenter as shown in FIG. 10, or projections 32 whose bottom surface shapehas a cross shape as shown in FIG. 11.

According to projections 31 whose bottom surface shape has a doglegshape shown in FIG. 10, since at least one side of the bottom surfaceincludes a depression in the center, a plurality of projections 31 canbe arranged so that a straight line passes through the inside of atleast any one of projections 31 when the straight line is drawn at anyposition in any direction on a surface of sapphire substrate on whichprojections 31 are formed.

Even in case of projections 32 (FIG. 11) whose bottom surface has across shape, each side of the bottom surface including a depression inthe center, a plurality of projections 32 can be arranged so that astraight line passes through the inside of at least any one ofprojections 32 when the straight line is drawn at any position in anydirection on a surface of sapphire substrate on which projections 32 areformed.

Embodiment 8

In a plan view of FIG. 12, the shape and the arrangement of projections40 in a sapphire substrate according to Embodiment 8 of the presentinvention are shown.

Projections 40 of Embodiment 8 are formed in the sense in whichprojections 1 of Embodiment 7 are rotated by 180 degrees. Herein, in thepresent description, the sense of projections is defined on the basis ofan orientation flat (plane A) 100 of a sapphire substrate as shown inFIG. 15.

That is, both projections 40 of Embodiment 8 and projections 1 ofEmbodiment 7 are formed so that the direction of projections (threedirections from the center toward the vertex of a substantiallytriangular shape in a bottom surface of the substantially triangularshape) and an axis “a” intersect with each other, but the direction ofprojections is opposite. As a result, the above three directions inprojections 1 of Embodiment 7 agree with those in which the axis “a”(axis a1, axis 2 and axis a3) shown in FIG. 15 is rotated counterclockwise (left-handed) by 30 degrees, whereas, the above threedirections in projections 40 of Embodiment 8 agree with those in whichthe axis “a” (axis a1, axis 2 and axis a3) shown in FIG. 15 is rotatedcounter clockwise (right-handed) by 30 degrees.

Projections 40 of Embodiment 8 in which projections 1 are rotated by 180degrees can be formed by forming a mask M45 extending in threedirections that agree with the above three directions shown in FIG. 13on a C-plane of a sapphire substrate, followed by etching.

As described above, the surface to be etched in the direction parallelto the axis “a” when rotated by 180 degrees makes an obtuse angle withthe C-plane, whereas, the surface to be etched in a vertical directionmakes an acute angle with the C-plane. Therefore, the shape ofprojections 40 formed by etching becomes the shape that is differentfrom that of projections 1, as described hereinafter.

The thus formed projections 40 of Embodiment 8 are the same asprojections 1 of Embodiment 7 in that a bottom surface has asubstantially triangular shape and also each side of a bottom surfacethereof includes a depression in the center. As is apparent from acomparison between FIG. 9 and FIG. 12, the proportion occupied by theinclined side 40 a based on the entire surface area in projections 40increases and the proportion occupied by the top surface 40 b relativelydecreases. In Embodiment 8, paying attention to the space between twoadjacent projections 40, an outer periphery shape of a protruded portionof one projection 40 has the same shape as that of the depressed side ofthe other projection 40, and a C-plane of a sapphire substrate is formedin almost the same width between the protruded portion of one projection40 and the depressed side of the other projection 40. Whereby, theC-plane of a sapphire substrate between three projections 1 extends in asubstantially triangular shape in Embodiment 7, whereas, the C-plane ofa sapphire substrate located between three projections 40 is formed inalmost the same width and becomes narrow in Embodiment 8. As describedabove, projections 40 of Embodiment 8 can widen an area of a bottomsurface of projections 40 as compared with an area of a bottom surfaceof projections 1 of Embodiment 7. As a result, an area of the C-plane ofa sapphire substrate between projections 40 can be decreased.

Accordingly, when a nitride semiconductor is grown on a surface of thesapphire substrate on which projections 40 of Embodiment 8 are formed,the proportion of the nitride semiconductor grown in a lateral directioncan be increased and dislocation of the grown nitride semiconductor canbe decreased.

As described above, in the sapphire substrate of Embodiment 8, each sideof a bottom surface of projections 40 respectively includes a depressionin the center and can be arranged so that a straight line passes throughthe inside of at least any one of projections 32 when the straight lineis drawn at any position in any direction on a surface of the sapphiresubstrate. In addition, the proportion of the nitride semiconductorgrown in a lateral direction can be increased and dislocation of thegrown nitride semiconductor can be decreased.

In the above Embodiment 8, as most preferable example, the above threedirections in projections 40 were allowed to agree with those in whichthe axis “a” (axis a1, axis 2 and axis a3) is rotated clockwise(right-handed) by 30 degrees. However, the present invention is notlimited thereto. For example, when the above three directions inprojections 40 are adjusted within a range of ±10 degrees of thedirection in which the axis “a” is rotated clockwise by 30 degrees, thesame operation and effect as in Embodiment 8 can be obtained.

Embodiment 9

In a plan view of FIG. 14, the shape and the arrangement of projections50 in a sapphire substrate according to Embodiment 9 of the presentinvention are shown.

Projections 50 in the sapphire substrate of Embodiment 9 are formed inthe sense in which projections 1 of Embodiment 7 are rotated by 180degrees, similar to projections 40 of Embodiment 8. However, theconstitution of a top surface 50 b of projections 50 is different fromthat of a top surface 40 b of projections 40 of Embodiment 8.Specifically, the top surface 50 b of Embodiment 9 is depressed in thecenter of projections 50 and the top surface 50 b is a surface inclinedfrom the sapphire C-plane.

In the sapphire substrate of Embodiment 9 including a plurality ofprojections 50 thus formed on one principal surface, the area of aC-plane of a sapphire substrate between projections 50 can be decreasedby increasing the proportion of the inclined side 50 a of projections50, and also the growth of a nitride semiconductor on the top surface 50b of projections 50, which is a surface inclined from the sapphireC-plane can be suppressed, similar to the sapphire substrate ofEmbodiment 8. Whereby, the sapphire substrate of Embodiment 9 canfurther decrease the area occupied by a C-plane based on the entire oneprincipal surface, and thus enabling the growth of a nitridesemiconductor with less threading dislocation. In addition, there arefollowing advantages.

That is, when the nitride semiconductor is grown on a top surface ofprojections, the height of projections increases and supply of a rawgas, that is usually supplied obliquely, is disturbed by projections,and thus pores (voids) are likely to generate at the root ofprojections. In contrast, like the present invention, when at least apart, preferably entirety of a top surface 50 b of projections 50 is asurface that is not a sapphire C-plane, the growth of a nitridesemiconductor on the top surface 50 b is suppressed, and thus thegeneration of pores (voids) can be suppressed. Furthermore, in thesapphire substrate of Embodiment 9, since a depression is formed in thecenter of a top surface of projections is formed, a raw gas sufficientlyreaches even the depression portion of a bottom surface of projections,and thus the generation of pores (voids) can be suppressed moreeffectively. Projections 50 including the top surface 50 b composed of asurface that is not a sapphire C-plane can be produced by appropriatelyadjusting the mask shape. In order to form the top surface 50 b with asurface that is not a sapphire C-plane without increasing an area of aC-plane of a sapphire substrate between projections 50, an intervalbetween masks may be appropriately adjusted.

Embodiment 10

In a plan view of FIG. 16A, the shape and the arrangement of projections60 in a sapphire substrate according to Embodiment 10 of the presentinvention are shown. In FIG. 16B, projections 60 of FIG. 16A areenlargely shown. As shown in FIG. 16A and FIG. 16B, projections 60include an inclined side 60 a and a top surface 60 b.

In this sapphire substrate of Embodiment 10, projections 60 are formedin the sense in which projections 1 of Embodiment 7 are rotatedclockwise by 30 degrees.

That is, as shown in FIG. 18( a) and FIG. 18( c), projections 60 ofEmbodiment 10 are formed so that three directions from the center towardthe vertex in a bottom surface of projections 60, each having asubstantially triangular shape, agree with the axis “a”. In a sapphiresubstrate of Embodiment 10, a plurality of projections 60 are arrangedin the direction that agrees with the axis “a”.

Projections 60 can be formed by forming a mask M65 formed so that threedirections from the center toward the tip agree with the axis “a” on aC-plane of a sapphire substrate in the direction that agrees with theaxis “a”, followed by etching. When the sapphire substrate is etchedusing the mask M65 thus formed, the direction that is easily etched andthe direction that is scarcely etched are formed (etching anisotropyappears). Therefore, as shown in FIG. 16A and FIG. 16B, a width A of aninclined side becomes larger than a width B of the other inclined sideat one side of the axis “a” and thus an area of the inclined side 60 aincreases. As a result, an area of a C-plane of a sapphire substrateformed between projections 60 can be decreased, and thus threadingdislocation in case of growing a nitride semiconductor can be decreased.

Furthermore, in the sapphire substrate of Embodiment 10, sinceprojections 60 are formed so that the extended portion extending inthree directions from the center toward the tip agrees with the axis“a”, a rate of the growth in a lateral direction, that coversprojections 60 of a nitride semiconductor grown from a C-plane betweenprojections 60, increases and flattening of the nitride semiconductor tobe grown is promoted, and thus it becomes possible to form a more flatnitride semiconductor layer

Describing in detail, the nitride semiconductor to be grown on asapphire C-plane slowly grows in the direction of the axis “a” andquickly grows in the direction rotated from the axis “a” by 30 degrees,as shown in FIG. 18( d). As a result, the nitride semiconductor in thedirection that intersects with the axis “a” grows more quickly.Accordingly, the extended portion extending in the direction that agreeswith the axis “a” of projections 60 is quickly covered with the nitridesemiconductor grown from both sides thereof (FIG. 18( c)).

As described above, according to the sapphire substrate of Embodiment10, it becomes possible to decrease threading dislocation in case ofgrowing a nitride semiconductor and to form a more flat nitridesemiconductor layer.

In the above Embodiment 10, as most preferable example, the above threedirections in projections 60 was allowed to agree with the axis “a”.However, the present invention is not limited thereto. For example, thesame operation and effect as in Embodiment 10 can be obtained byadjusting the above three directions in projections 60 within a range ofthe axis “a”±10 degrees.

Embodiment 11

In a plan view of FIG. 19, the shape and the arrangement of projections70 in a sapphire substrate according to Embodiment 11 of the presentinvention is shown.

Projections 70 in the sapphire substrate of Embodiment 11 is composed ofa subprojection 70 s 1, a subprojection 70 s 2 and a subprojection 70 s3 in the form where projections 1 having a petal shape in Embodiment 7are separated in the center. When projections 70 are separated in thecenter, the portion of a triangular shape through which light is capableof going straight is formed between subprojections 70 s 1, 70 s 2, 70 s3 on one principal surface of the sapphire substrate of Embodiment 11,and three subprojections 70 s 1, 70 s 2, 70 s 3 are proximal to eachother. Therefore, even in case of the constitution of projections 70 inEmbodiment 11, similar to projections 1, each having a petal shape, inEmbodiment 7, projections 70 can be arrange so that a straight linepasses through the inside of at least any one of projections 70 when thestraight line is drawn at any position in any direction on a surface ofa sapphire substrate on which projections 70 are formed.

Projections 70 separated in the center can be formed by forming a mask70 composed of masks M70 a 1, M70 a 2, M70 a 3 separated in the centeras shown in FIG. 20 in the predetermined arrangement and etching asapphire substrate. The shape of the portion to be separated can beadjusted by a shape of tip portions facing each other of masks M70 a 1,M70 a 2, M70 a 3 as shown in FIG. 20.

In the sapphire substrate constituted as described above of Embodiment11, projections 70 separated into subprojections 70 s 1, 70 s 2, 70 s 3are formed on one principal surface. Whereby, when a nitridesemiconductor is grown on a sapphire C-plane on which projections 70 areformed, the occurrence of voids can be prevented without suppression ofthe supply of a raw gas in the center of projections 70.

Embodiment 12

In a plan view of FIG. 21, the shape and the arrangement of projections80 in a sapphire substrate according to Embodiment 12 of the presentinvention are shown.

Projections 80 in the sapphire substrate of Embodiment 12 is composed ofa subprojection 80 s 1, a subprojection 80 s 2 and a subprojection 80 s3 in the form where projections 40 having a petal shape in Embodiment 8are separated in the center. When projections 80 are separated in thecenter, the gap through which light is capable of going straight isformed between subprojections 80 s 1, 80 s 2, 80 s 3 on one principalsurface of the sapphire substrate of Embodiment 12, and threesubprojections 80 s 1, 80 s 2, 80 s 3 are proximal to each other.Therefore, even in case of the constitution of projections 80 inEmbodiment 12, similar to projections 40, each having a petal shape, inEmbodiment 8, projections 80 can be arrange so that a straight linepasses through the inside of at least any one of projections 80 when thestraight line is drawn at any position in any direction on a surface ofa sapphire substrate on which projections 40 are formed.

Projections 80 separated in the center can be formed by forming a mask70 composed of masks M80 a 1, M80 a 2, M80 a 3 separated in the centeras shown in FIG. 22 in the predetermined arrangement and etching asapphire substrate. The reason why the shape of the portion to beseparated is different from that in Embodiment 11 shown in FIG. 19 isdue to direction anisotropy of an etching rate of the sapphire substratesince projections 80 of Embodiment 12 are formed in the sense in whichprojections 70 of Embodiment 11 are rotated by 180 degrees.

In the sapphire substrate constituted as described above of Embodiment12, projections 80 separated into subprojections 80 s 1, 80 s 2, 80 s 3are formed on one principal surface. Whereby, when a nitridesemiconductor is grown on a sapphire C-plane on which projections 80 areformed, the occurrence of voids can be prevented without suppression ofthe supply of a raw gas in the center of projections 80.

BRIEF DESCRIPTION OF REFERENCE NUMERALS

-   1, 2, 31, 32, 40, 50, 60, 70, 80: Projection-   1 a, 40 a, 50 a, 60 a, 70 a, 80 a: Inclined side of projection-   1 b, 40 b, 50 b, 60 b, 70 b, 80 b: Top surface of projection-   10: Sapphire substrate-   11, 12, 13: Side of bottom surface of projection-   11 a, 11 b, 12 a, 12 b, 13 a, 13 b: Curved line-   20: Semiconductor laminated structure-   21: Base layer-   22: First conductive layer (n-type layer)-   23: Active layer (emissive layer)-   24: Second conductive layer (p-type layer)-   M1, M45, M65, M70, M80: Mask-   70 s 1, 70 s 2, 70 s 3, 80 s 1, 80 s 2, 80 s 3: Subprojection-   100: Orientation flat

1. A sapphire substrate having a principal surface for growing a nitride semiconductor to form a nitride semiconductor light emitting device, the sapphire substrate comprising a plurality of projections on the principal surface, wherein each of the projections has a bottom that has a substantially polygonal shape, wherein each side of the bottom of the projections has a depression in its center, and wherein vertexes of the bottoms of the respective projections extend in a direction that is within a range of ±10 degrees of a direction that is rotated clockwise by 30 degrees from a crystal axis “a” of the sapphire substrate.
 2. The sapphire substrate according to claim 1, wherein the plurality of projections are arranged so that any straight line that is drawn at any position in any direction in a plane including the bottom surfaces of the plurality of projections passes through the inside of at least one of projections.
 3. The sapphire substrate according to claim 1, wherein each of the projections has a substantially polygonal pyramid shape or substantially truncated polygonal pyramidal shape.
 4. The sapphire substrate according to claim 1, wherein the plurality of projections are arranged so that one of vertexes of the bottom of one projection of neighboring projections is located in a region defined by connecting points of two vertexes of the bottom and the deepest point of the depression of the other projection.
 5. The sapphire substrate according to claim 4, wherein said one projection of the neighboring projections is arranged so that the deepest portion of a depression of the other projection is located on an extended line of a line that bisect an angle of a vertex of said one projection.
 6. The sapphire substrate according to claim 1, wherein each of projections has a shape with linear symmetry to a straight line that connects a depression and a vertex opposite to the side including the depression.
 7. The sapphire substrate according to claim 1, wherein each of the projections has a top surface that has a shape substantially similar to that of the bottom.
 8. The sapphire substrate according to claim 1, wherein each of the projections has a bottom in a shape of substantially triangle.
 9. The sapphire substrate according to claim 8, wherein the plurality of projections are arranged so as to form a plurality of rows, and wherein the plurality of projections are arranged so that bisectors that bisect an angle of one of vertexes of projections arranged in the same row are located on a straight line, and projection directions of the projections in the same row, defined by a direction of the bisector that is drawn from an inside to outside of the projections, become the same.
 10. The sapphire substrate according to claim 9, wherein a direction of the projections forming one row of neighboring rows is opposite to a direction of the projections forming the other of the neighboring rows.
 11. The sapphire substrate according to claim 1, wherein a group composed of (n+1) number of projections located at vertexes and a center of a substantially regular n-sided polygon, is repeatedly arranged.
 12. The sapphire substrate according to claim 11, wherein the group shares a part of the projections with neighboring groups.
 13. The sapphire substrate according to claim 8, wherein each of the projections comprise three sub-projections elongating in three directions from the center of the projection and the sub-projections are separated with each other at a center portion that includes the center of the projection.
 14. A semiconductor light emitting device that is a nitride semiconductor light emitting device formed by growing a nitride semiconductor layer on one principal surface of the sapphire substrate according to claim
 1. 